In wireless communication networks for radio communication with user-controlled terminals or devices, it is often possible to select different transmission modes for transmitting downlink signals from a radio network node depending on the current circumstances. Such transmission modes may differ from one another in terms of, e.g., antenna arrangements, modulation/demodulation schemes, encoding schemes, beam forming, spatial multiplexing, transmit diversity, and so forth. In this disclosure, the term “User Equipment, UE” will be used to represent any wireless terminal or device capable of radio communication, including receiving downlink signals transmitted from a radio network node of a wireless communication network. The radio network node may be any node of a wireless communication network that can transmit downlink radio signals to UEs. The radio network node in this description may also be referred to as a base station, NodeB, e-NodeB, eNB, base transceiver station, etc.
For example, one transmission mode may be preferable to use and provide better performance to the downlink communication in one situation while another transmission mode may be better to use in another situation. In this context, the performance of a communication may be represented by achieved throughput, i.e. the amount of data communicated per time unit which may be measured in bits per second, bps. These situations typically refer to different conditions of a used radio channel which are highly dependent of the geographical location of the UE. In general, when a UE is located relatively close to the transmitting radio network node the UE experiences better channel conditions and signal quality than when the UE is located relatively far away from the radio network node, e.g. close to a cell edge, although other factors may also influence the channel conditions and signal quality such as occurrence of obstacles and signal reflections, which may be different, e.g., in rural and urban environments.
In current solutions, the signal quality is a factor of great importance to consider when selecting which transmission mode to use, and there are some well-known measurable parameters that may be used as indication of signal quality, such as Signal to Noise Ratio, SNR, Signal to Noise and Interference Ratio, SINR, Reference Signal Received Quality, RSRQ, and Channel Quality Indicator, CQI. Values of these parameters may be obtained on a continuous basis from measurements made by the UE and/or estimations made by the network, by means of well-known procedures which are not necessary to describe here in any detail.
In Long Term Evolution, LTE, Release 8, seven Transmission Modes, TMs, are defined for downlink, denoted as TM1-TM7. For example, TM2 is a “robust” mode and provides transmit diversity, and TM2 is often used as a fallback mode in situations of very poor signal propagation and quality due to, e.g., high path loss, severe interference, high terminal speed, etc. An antenna configuration with 8 physical-ports antenna is deployed in some LTE networks due to its high uplink receiving performance and downlink beam forming performance. In this case, either of transmission modes TM3 and TM7 is suitable to use for downlink data transmission depending on current signal quality. Of these modes, TM7 is a single-layer Multiple Input Multiple Output, MIMO, scheme where the 8-ports antenna may be used to form beams directed to the UE to achieve better coverage and higher downlink throughput than other transmission modes when the UE is located relatively far away from the transmitting radio network node. On the other hand, TM3 is an open loop MIMO scheme and provides better performance, such as downlink throughput, when the UE is located relatively close to the transmitting radio network node.
It can be readily understood that the signal quality may change rapidly, e.g. when the UE moves around, and ideally the transmission mode should be switched accordingly as follows, to achieve optimal performance. The obtained indication of signal quality may be used for evaluating a threshold condition such that a first transmission mode is selected when the signal quality is above a certain switching threshold while a second transmission mode is selected when the signal quality is below the switching threshold. A margin may be used in the threshold condition to avoid frequent switching back and forth between the transmission modes in a “ping-pong” manner when the signal quality fluctuates around the switching threshold. An example of a procedure for applying the above threshold condition in a radio network node for sending downlink signals to a UE is illustrated by a flow chart in FIG. 1.
In a first shown action 100, the radio network node sends downlink signals to the UE using the first transmission mode which may be a default transmission mode that is used initially when the radio communication starts, i.e. before any signal quality has been obtained for that UE. In a next action 102, the radio network node monitors and obtains a signal quality of the downlink signals transmitted to the UE using the first transmission mode, e.g. from measurements made by the UE and/or estimations made by the network. It is then determined in an action 104 whether the obtained signal quality is, in this example, below the switching threshold, e.g. by a margin as mentioned above. If not, the radio network node continues to use the first transmission mode thus returning to action 100. On the other hand, if the signal quality is below the switching threshold, the radio network node sends downlink signals to the UE using the second transmission mode instead, in an action 106.
In a further action 108, the radio network node continues to monitor and obtain signal quality when using the second transmission mode. It is then determined in an action 110 whether the obtained signal quality has improved and is, in this example, above the switching threshold, e.g. by a margin which may be equal to or different than the margin mentioned in action 104 above. If the switching threshold is not exceeded, the radio network node continues to use the second transmission mode thus returning to action 106. On the other hand, if the signal quality is above the switching threshold, the radio network node switches transmission mode and sends signals to the UE using the first transmission mode again, by returning to action 100. In practice, it may be required that the conditions of actions 104 and 110 must be fulfilled for a minimum time before switching transmission mode. Further, it may be required that a new transmission mode has been used for a minimum time before switching back again to the previous mode.
FIG. 2 is a diagram illustrating curves with measurements of a resulting downlink throughput at different signal qualities, here indicated by the parameter SNR in decibel, dB. The curve with circular points shows the throughput when using TM3 as the first transmission mode, and the curve with square points shows the throughput when using TM7 as the second transmission mode. The downlink throughput in this diagram is measured in bits per second, bps, ×106. It can be seen that TM7 provides higher throughput than TM3 at relatively low SNR values, while TM3 provides higher throughput than TM7 at relatively high SNR values. Ideally, the switching threshold should in this example be around an SNR of 15 dB such that TM7 is used when SNR<15 dB and TM3 is used when SNR>15 dB, which would provide the highest possible throughput regardless of SNR.
As mentioned above, a threshold condition for determining which transmission mode to use may be defined as follows:                a first transmission mode is used when a quality of downlink signals received by a UE is above a switching threshold, possibly by a margin, and        a second transmission mode is used when the quality of downlink signals received by the UE is below the switching threshold, possibly by another margin which may be the same as or different than the previous margin.When such margins are used, the threshold condition could alternatively be defined by a first and a second threshold, respectively, the first threshold being somewhat higher than the second threshold.        
However, it is a problem that the switching threshold is sometimes not optimal such that the best possible performance is not achieved at some quality levels. For example, when the switching threshold is too high, the second transmission mode is over-estimated and will be used even at a signal quality when the first transmission mode would actually provide better performance, i.e. throughput. Conversely, when the switching threshold is too low, the first transmission mode is over-estimated and will be used even at a signal quality when the second transmission mode would actually provide better performance. An incorrect setting of the switching threshold may thus result in deteriorated performance in downlink transmission to a UE.